Utils: Join SynchronousProcess::run and runBlocking implementations

Make functionality dependent on an (intentionally ugly)
setProcessUserEventWhileRunning call.

Also, back-paddle a bit on API combination of QtcProcess and
SynchronousPrceoss for now and prevent the QtcProcess-and-
runBlocking and SynchronousProcess-and-start combinations.

Goal is still to have all in QtcProcess in the end, but this
may take a while.

Change-Id: Ic146ec5db0ab8dc9613e5b2af5f4dc90bc7465ca
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/clearcase/clearcaseplugin.cpp

@@ -2359,7 +2359,8 @@ QString ClearCasePluginPrivate::runExtDiff(const QString &workingDir, const QStr
     process.setWorkingDirectory(workingDir);
     process.setCodec(outputCodec ? outputCodec : QTextCodec::codecForName("UTF-8"));
     process.setCommand(diff);
-    process.run();
+    process.setProcessUserEventWhileRunning();
+    process.runBlocking();
     if (process.result() != QtcProcess::Finished)
         return QString();
     return process.allOutput();